Many different methods for the preparation of epoxides have been developed. One such method involves the liquid-phase epoxidation of an olefin with an organic hydroperoxide in the presence of a solubilized transition metal catalyst. Although highly active and selective for olefin epoxidation, soluble catalysts must be recovered or recycled after use to avoid loss to a waste stream. However, it can be very expensive to recover the soluble catalysts after use. In addition, recycle decreases catalyst productivity by also recycling certain heavy substances such as acids and polymers that tend to accumulate along with catalyst in the heavy bottoms stream. The recycled heavies' stream decreases epoxide selectivity or olefin conversion.
Heterogeneous (insoluble) catalysts have been developed to avoid homogeneous catalyst disadvantages. U.S. Pat. No. 4,367,342 and British Pat. No. 1,332,527 disclose an olefin epoxidation process in the presence of an insoluble titania-silica catalyst comprised of an inorganic oxygen compound of titanium. Unfortunately, the disclosed catalysts have less than optimum activity and selectivity. A later-filed patent application (EP 345,856) discloses the preparation of epoxidation catalysts that are alleged to be more active than the analogous catalysts obtained by previously known procedures. EP 345,856 teaches impregnation of silica with a gaseous stream of titanium tetrachloride, followed by calcination, hydrolysis, and, optionally, silylation. Additionally, U.S. Pat. Nos. 6,011,162 and 6,114,552 disclose catalysts prepared by a liquid-phase impregnation process in a non-oxygen containing solvent.
Unfortunately, heterogeneous catalysts of the type disclosed above tend to slowly deteriorate in performance when used repeatedly or in a continuous process for a prolonged period of time. In particular, the catalyst activity (as measured by the amount of olefin or organic hydroperoxide converted per pass or in a given period of time) decreases with time to a point where continued use of the catalyst charge is no longer economically viable. Due to the relatively high cost of synthesizing this type of catalyst, regeneration of the used catalyst would be greatly preferred over replacement.
U.S. Pat. No. 5,798,313 discloses a method of restoring the activity of a used titanium-containing silicon oxide catalyst by heating the spent catalyst in the presence of a gas stream comprised of oxygen at a temperature of at least 700° C. In addition, U.S. Pat. No. 5,916,835 discloses a method of restoring the activity of a used titanium-containing silicon oxide catalyst by contacting the spent heterogeneous catalyst with one or more specific types of solvents, preferably at a moderately elevated temperature.
As with any chemical process, it is desirable to develop new and improved regeneration methods. We have discovered an effective regeneration method to restore the activity of a used titanium-containing silicon oxide catalyst.